1. Field of the Disclosure
The present disclosure relates generally to an optical scanning system in an imaging apparatus, and particularly to synchronization optics used in a scanning unit.
2. Description of the Related Art
In various imaging devices which utilize light to form images, optical scanning systems are typically incorporated to scan light beams from one or more light sources onto a target image plane surface. In an electrophotographic imaging device, for example, the image plane surface is typically a photosensitive member. Generally, light beams are swept across the image plane surface by a scanning mirror to form light spots upon the image plane surface along a scan line direction. The scanning mirror may be a rotating polygon mirror which scans light beams in one direction, or an oscillating mirror which scans light beams bidirectionally in both forward and reverse directions. Multiple scan lines are formed as light beams are scanned in a process/sub-scan direction, such as when the image plane surface moves orthogonally relative to the scan line direction while the scanning mirror is scanning the light beams in the scan line direction.
In order to achieve accurate writing of image information on the image plane surface, it is known to synchronize the formation of the scan lines on the target image plane surface. Typically, synchronization is achieved by optically detecting light beams with one or more photodetectors at the start of each scan line operation. Light beams detected by the photodector create pulses that are used to synchronize the start of scan for each successive scan line such that the scan lines start at a common reference. In some scanning systems, the photodetector is disposed downstream of the scanning mirror to directly intercept a light beam scanned proximate a boundary that the scanning mirror can scan the light beam. For bidirectional scanners, another photodetector may be disposed to directly intercept the light beam when scanned near an opposite scanning boundary. In other existing designs, mirrors are disposed to intercept a light beam when scanned near the scan boundaries, and to direct the intercepted light beam towards a photodetector.
In order to focus light beams on the photodetector, some existing approaches have taken advantage of the focusing function of scan lenses which are used to focus light beams onto the image plane surface. For example, as shown in FIG. 1 illustrating an example scanning unit 10, a sensor 15 for sensing a light beam 20 when scanned by a scanning mirror 25 at opposite scan boundaries represented as virtual extensions 30A, 30B of light beam 20, is disposed downstream of a scan lens 35, relative to optical paths of the light beam 20. In this way, light beam 20 first passes through scan lens 35 and is focused thereby before reaching sensor 15. However, in order to focus light beam 20 on sensor 15, the optical path length of light beam 20 from scanning mirror 25 to sensor 15 is substantially matched with the optical path length of light beam 20 from scanning mirror 25 to the image plane surface 37. Because of the optical length constraints, mirrors 40A, 40B, and 40C are typically emplaced within the housing of the scanning unit 10 and downstream scan lens 35 to intercept and direct light beam 20 toward sensor 15. In particular, mirrors 40A and 40C pick off light beam 20 at the scan boundaries 30A and 30B, respectively, after light beam 20 passes through scan lens 35. The light beam 20 picked off by mirror 40C is directly reflected towards sensor 15 while the light beam 20 picked off by mirror 40A is directed to and reflected by mirror 40B before reaching sensor 15. This, however, increases the overall size of the housing of the scanning unit 10.
Moreover, some existing designs also incorporate additional focusing lenses placed in the optical path of a light beam deflected toward the photodetector. For example, in FIG. 1, a synchronization lens 45 is disposed in front of sensor 15 to focus light beams directed by mirrors 40 toward sensor 15. Generally, such synchronization lenses are cylindrical. In addition, since adding optical power along the scan line direction may significantly reduce laser spot velocity and increase scan jitter, these cylindrical lenses typically have optical power only along the process direction.
In the above example designs, the size and number of synchronization optical components, the complex beam paths, and constraints on optical path length, all serve to increase the size of the housing of the scanning unit and, consequently, increase the overall cost of the housing of the scanning unit.
Accordingly, there is a need for a scanning unit implementing a simplified synchronization optics design to improve compactness and cost of the scanning unit.